Civil Engineering Reference
In-Depth Information
Figure 3.20
Surfaces of potential failure in longitudinal shear
Figure 3.21
T-beam with asymmetrical concrete flange
The word 'surface' is used here because EFGH in Fig. 3.20, although
not a plane, is another potential surface of shear failure. In practice, the rules
for minimum height of shear connectors ensure that in slabs of uniform
thickness, surfaces of type B-B are more critical; but this may not be so
for haunched slabs, considered later.
The design longitudinal shear per unit length ('shear flow', denoted
v
L
)
for surface EFGH is almost the same as that for the shear connection, and
in a symmetrical T-beam half of that value is assumed to be transferred
through each of the planes B-B and D-D. For an L-beam (Fig. 3.21) or
where the flange of the steel beam is wide, the more accurate expressions
should be used:
v
L,BB
=
v
L
b
1
/
b
and
v
L,DD
=
v
L
b
2
/
b
(3.74)
where
v
L
is the design shear flow for the shear connection and
b
is the
effective width of the concrete flange.
Effective area of reinforcement
For planes such as B-B in Fig. 3.20, the effective area of transverse
reinforcement per unit length of beam,
A
e
, is the whole of the reinforce-
ment that is fully anchored on both sides of the plane (i.e., able to develop
its yield strength in tension). This is so even where the top bars are fully
stressed by the bending moment
M
s
, because this tension is balanced by
